It has heretofore been common practice to drive injection molding machines hydraulically. This has required expensive, large, long-stroke cylinders to provide the travel and high force required. Such cylinders have also required accompanying high-pressure, high-flow, hydraulic power supplies, and servovalves that could control flow and pressure over a wide range.
Lower cost, but still high-performance, electro-mechanical drives using brushless DC servomotors are now replacing hydraulics in many industrial applications. However, the injection molding application has proven especially difficult for this transition because of the need for a combination of controlled velocity under varying loads (to which electro-mechanical drives are well-suited), and the precisely-programmed build-up of high force at extremely low velocities during the mold packing phase of the process (to which the hydraulic drive is well-suited).
The present invention addresses this need by combining the best features of both electro-mechanical and hydraulic drives, but without requiring an expensive hydraulic power supply. This is accomplished by arranging an electro-mechanical screw drive actuator in series with a hydraulic actuator, so that the resulting hybrid electro-mechanical-hydraulic actuator thus formed can operate at a low, controlled velocity to develop tile hydraulic pressure, which is in turn controlled by a servovalve.
The prior art is believed to contain many examples which combine electro-mechanical and hydraulic drives, either in series or parallel with one another. Many of these are believed to be used for press applications. Typically, the electric drive is used to provide rapid advance at low load to a working position, at which point pressure from a pump is applied to a hydraulic cylinder to create high force with very little movement. Upon information and belief, the prior art does not contemplate the use of the electro-mechanical drive to create the pressure required by the hydraulic actuator.